Sodium (Na) is an alkali metal. It belongs to group 1, and all the elements in group 1 have one valence electron.
Sodium has an electron configuration of:
The valence electron is the one electron located in the 3s sublevel.
Here is a video to help with this concept:
video from: Noel Pauller
As you know, valence electrons are the electrons located in the outermost shell of an atom. If you're familiar with electron configurations, you can easily see that this is the case by examining the number of electrons sodium has in its outermost shell.
So, sodium's outermost shell, which is sometimes referred to as the valence shell, is 3s. Since this shell is occupied by just one electron, that's how many valence electrons neutral sodium will have
Notice that the sodium cation is formed when that single valence electron is removed from the 3s shell, which allows sodium to have a full octet.